PRECIS                                                    P. Saint-Andre
Internet-Draft                                                      &yet
Obsoletes: 3454 (if approved)                                M. Blanchet
Intended status: Standards Track                                Viagenie
Expires: March 6, 2015                                 September 2, 2014


   PRECIS Framework: Preparation and Comparison of Internationalized
                    Strings in Application Protocols
                     draft-ietf-precis-framework-18

Abstract

   Application protocols using Unicode characters in protocol strings
   need to properly prepare such strings in order to perform valid
   comparison operations (e.g., for purposes of authentication or
   authorization).  This document defines a framework enabling
   application protocols to perform the preparation and comparison of
   internationalized strings ("PRECIS") in a way that depends on the
   properties of Unicode characters and thus is agile with respect to
   versions of Unicode.  As a result, this framework provides a more
   sustainable approach to the handling of internationalized strings
   than the previous framework, known as Stringprep (RFC 3454).  This
   document obsoletes RFC 3454.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 6, 2015.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  String Classes  . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  IdentifierClass . . . . . . . . . . . . . . . . . . . . .   7
       3.2.1.  Valid . . . . . . . . . . . . . . . . . . . . . . . .   7
       3.2.2.  Contextual Rule Required  . . . . . . . . . . . . . .   8
       3.2.3.  Disallowed  . . . . . . . . . . . . . . . . . . . . .   8
       3.2.4.  Unassigned  . . . . . . . . . . . . . . . . . . . . .   9
       3.2.5.  Examples  . . . . . . . . . . . . . . . . . . . . . .   9
     3.3.  FreeformClass . . . . . . . . . . . . . . . . . . . . . .   9
       3.3.1.  Valid . . . . . . . . . . . . . . . . . . . . . . . .   9
       3.3.2.  Contextual Rule Required  . . . . . . . . . . . . . .  10
       3.3.3.  Disallowed  . . . . . . . . . . . . . . . . . . . . .  10
       3.3.4.  Unassigned  . . . . . . . . . . . . . . . . . . . . .  10
       3.3.5.  Examples  . . . . . . . . . . . . . . . . . . . . . .  10
   4.  Profiles  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Principles  . . . . . . . . . . . . . . . . . . . . . . .  11
       4.1.1.  Width Mapping . . . . . . . . . . . . . . . . . . . .  11
       4.1.2.  Additional Mappings . . . . . . . . . . . . . . . . .  12
       4.1.3.  Case Mapping  . . . . . . . . . . . . . . . . . . . .  12
       4.1.4.  Normalization . . . . . . . . . . . . . . . . . . . .  12
       4.1.5.  Directionality  . . . . . . . . . . . . . . . . . . .  12
       4.1.6.  Exclusions  . . . . . . . . . . . . . . . . . . . . .  13
     4.2.  Building Application-Layer Constructs . . . . . . . . . .  13
     4.3.  A Note about Spaces . . . . . . . . . . . . . . . . . . .  14
   5.  Order of Operations . . . . . . . . . . . . . . . . . . . . .  14
   6.  Code Point Properties . . . . . . . . . . . . . . . . . . . .  15
   7.  Category Definitions Used to Calculate Derived Property . . .  18
     7.1.  LetterDigits (A)  . . . . . . . . . . . . . . . . . . . .  18
     7.2.  Unstable (B)  . . . . . . . . . . . . . . . . . . . . . .  18
     7.3.  IgnorableProperties (C) . . . . . . . . . . . . . . . . .  19
     7.4.  IgnorableBlocks (D) . . . . . . . . . . . . . . . . . . .  19
     7.5.  LDH (E) . . . . . . . . . . . . . . . . . . . . . . . . .  19
     7.6.  Exceptions (F)  . . . . . . . . . . . . . . . . . . . . .  19
     7.7.  BackwardCompatible (G)  . . . . . . . . . . . . . . . . .  19



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     7.8.  JoinControl (H) . . . . . . . . . . . . . . . . . . . . .  19
     7.9.  OldHangulJamo (I) . . . . . . . . . . . . . . . . . . . .  20
     7.10. Unassigned (J)  . . . . . . . . . . . . . . . . . . . . .  20
     7.11. ASCII7 (K)  . . . . . . . . . . . . . . . . . . . . . . .  20
     7.12. Controls (L)  . . . . . . . . . . . . . . . . . . . . . .  20
     7.13. PrecisIgnorableProperties (M) . . . . . . . . . . . . . .  20
     7.14. Spaces (N)  . . . . . . . . . . . . . . . . . . . . . . .  21
     7.15. Symbols (O) . . . . . . . . . . . . . . . . . . . . . . .  21
     7.16. Punctuation (P) . . . . . . . . . . . . . . . . . . . . .  21
     7.17. HasCompat (Q) . . . . . . . . . . . . . . . . . . . . . .  21
     7.18. OtherLetterDigits (R) . . . . . . . . . . . . . . . . . .  21
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
     8.1.  PRECIS Derived Property Value Registry  . . . . . . . . .  21
     8.2.  PRECIS Base Classes Registry  . . . . . . . . . . . . . .  22
     8.3.  PRECIS Profiles Registry  . . . . . . . . . . . . . . . .  22
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  24
     9.1.  General Issues  . . . . . . . . . . . . . . . . . . . . .  24
     9.2.  Use of the IdentifierClass  . . . . . . . . . . . . . . .  25
     9.3.  Use of the FreeformClass  . . . . . . . . . . . . . . . .  25
     9.4.  Local Character Set Issues  . . . . . . . . . . . . . . .  26
     9.5.  Visually Similar Characters . . . . . . . . . . . . . . .  26
     9.6.  Security of Passwords . . . . . . . . . . . . . . . . . .  28
   10. Interoperability Considerations . . . . . . . . . . . . . . .  29
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  29
     11.2.  Informative References . . . . . . . . . . . . . . . . .  30
     11.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  32
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

1.  Introduction

   As described in the problem statement for the preparation and
   comparison of internationalized strings ("PRECIS") [RFC6885], many
   IETF protocols have used the Stringprep framework [RFC3454] as the
   basis for preparing and comparing protocol strings that contain
   Unicode characters [Unicode7.0] outside the ASCII range [RFC20].  The
   Stringprep framework was developed during work on the original
   technology for internationalized domain names (IDNs), here called
   "IDNA2003" [RFC3490], and Nameprep [RFC3491] was the Stringprep
   profile for IDNs.  At the time, Stringprep was designed as a general
   framework so that other application protocols could define their own
   Stringprep profiles for the preparation and comparison of strings and
   identifiers.  Indeed, a number of application protocols defined such
   profiles.

   After the publication of [RFC3454] in 2002, several significant
   issues arose with the use of Stringprep in the IDN case, as



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   documented in the IAB's recommendations regarding IDNs [RFC4690]
   (most significantly, Stringprep was tied to Unicode version 3.2).
   Therefore, the newer IDNA specifications, here called "IDNA2008"
   ([RFC5890], [RFC5891], [RFC5892], [RFC5893], [RFC5894]), no longer
   use Stringprep and Nameprep.  This migration away from Stringprep for
   IDNs has prompted other "customers" of Stringprep to consider new
   approaches to the preparation and comparison of internationalized
   strings, as described in [RFC6885].

   This document defines a framework for a post-Stringprep approach to
   the preparation and comparison of internationalized strings in
   application protocols, based on several principles:

   1.  Define a small set of string classes that specify the Unicode
       characters (i.e., specific "code points") appropriate for common
       application protocol constructs.

   2.  Define each PRECIS string class in terms of Unicode code points
       and their properties so that an algorithm can be used to
       determine whether each code point or character category is (a)
       valid, (b) allowed in certain contexts, (c) disallowed, or (d)
       unassigned.

   3.  Use an "inclusion model" such that a string class consists only
       of code points that are explicitly allowed, with the result that
       any code point not explicitly allowed is forbidden.

   4.  Enable application protocols to define profiles of the PRECIS
       string classes, addressing matters such as width mapping, case
       folding and other forms of character mapping, Unicode
       normalization, directionality, and further excluded code points
       or character categories.

   Whereas the string classes define the "baseline" code points for a
   range of applications, profiling enables application protocols to
   further restrict the allowable code points beyond those specified for
   the relevant string class (e.g., characters with special or reserved
   meaning, such as "@" and "/" when used as separators within
   identifiers) and to apply the string classes in ways that are
   appropriate for constructs such as usernames and passwords
   [I-D.ietf-precis-saslprepbis], nicknames [I-D.ietf-precis-nickname],
   the localparts of instant messaging addresses
   [I-D.ietf-xmpp-6122bis], and free-form strings
   [I-D.ietf-xmpp-6122bis].  Profiles are responsible for defining the
   handling of right-to-left characters as well as various mapping
   operations of the kind also discussed for IDNs in [RFC5895], such as
   case preservation or lowercasing, Unicode normalization, mapping of




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   certain characters to other characters or to nothing, and mapping of
   full-width and half-width characters.

   When an application applies a profile of a PRECIS string class, it
   can achieve the following objectives:

   a.  Determine if a given string conforms to the profile (e.g. to
       determine if it is allowed for use in the relevant "slot"
       specified by an application protocol).

   b.  Determine if any two given strings are equivalent (e.g., to make
       an access decision for purposes of authentication or
       authorization as further described in [RFC6943]).

   It is expected that this framework will yield the following benefits:

   o  Application protocols will be agile with regard to Unicode
      versions.

   o  Implementers will be able to share code point tables and software
      code across application protocols, most likely by means of
      software libraries.

   o  End users will be able to acquire more accurate expectations about
      the characters that are acceptable in various contexts.  Given
      this more uniform set of string classes, it is also expected that
      copy/paste operations between software implementing different
      application protocols will be more predictable and coherent.

   Although this framework is similar to IDNA2008 and borrows some of
   the character categories defined in [RFC5892], it defines additional
   character categories to meet the needs of common application
   protocols.

   The character categories and calculation rules defined under
   Section 7 and Section 6 are normative and apply to all Unicode code
   points.  The code point table that results from applying the
   character categories and calculation rules to the latest version of
   Unicode are provided in an IANA registry.

2.  Terminology

   Many important terms used in this document are defined in [RFC5890],
   [RFC6365], [RFC6885], and [Unicode7.0].  The terms "left-to-right"
   (LTR) and "right-to-left" (RTL) are defined in Unicode Standard Annex
   #9 [UAX9].





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   As of the date of writing, the version of Unicode published by the
   Unicode Consortium is 6.3 [Unicode7.0]; however, PRECIS is not tied
   to a specific version of Unicode.  The latest version of Unicode is
   always available [UnicodeCurrent].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

3.  String Classes

3.1.  Overview

   Starting in 2010, various "customers" of Stringprep began to discuss
   the need to define a post-Stringprep approach to the preparation and
   comparison of internationalized strings other than IDNs.  This
   community analyzed the existing Stringprep profiles and also weighed
   the costs and benefits of defining a relatively small set of Unicode
   characters that would minimize the potential for user confusion
   caused by visually similar characters (and thus be relatively "safe")
   vs. defining a much larger set of Unicode characters that would
   maximize the potential for user creativity (and thus be relatively
   "expressive").  As a result, the community concluded that most
   existing uses could be addressed by two string classes:

   IdentifierClass:  a sequence of letters, numbers, and some symbols
      that is used to identify or address a network entity such as a
      user account, a venue (e.g., a chatroom), an information source
      (e.g., a data feed), or a collection of data (e.g., a file); the
      intent is that this class will minimize user confusion in a wide
      variety of application protocols, with the result that safety has
      been prioritized over expressiveness for this class.

   FreeformClass:  a sequence of letters, numbers, symbols, spaces, and
      other characters that is used for free-form strings, including
      passwords as well as display elements such as human-friendly
      nicknames in chatrooms; the intent is that this class will allow
      nearly any Unicode character, with the result that expressiveness
      has been prioritized over safety for this class (e.g., protocol
      designers, application developers, service providers, and end
      users might not understand or be able to enter all of the
      characters that can be included in the FreeformClass - see
      Section 9.3 for details).

   Future specifications might define additional PRECIS string classes,
   such as a class that falls somewhere between the IdentifierClass and
   the FreeformClass.  At this time, it is not clear how useful such a



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   class would be.  In any case, because application developers are able
   to define profiles of PRECIS string classes, a protocol needing a
   construct between the IdentiferClass and the FreeformClass could
   define a restricted profile of the FreeformClass if needed.

   The following subsections discuss the IdentifierClass and
   FreeformClass in more detail, with reference to the dimensions
   described in Section 3 of [RFC6885].  Each string class is defined by
   the following behavioral rules:

   Valid:  Defines which code points and character categories are
      treated as valid input to the string.

   Contextual Rule Required:  Defines which code points and character
      categories are treated as allowed only if the requirements of a
      contextual rule are met (i.e., either CONTEXTJ or CONTEXTO).

   Disallowed:  Defines which code points and character categories need
      to be excluded from the string.

   Unassigned:  Defines application behavior in the presence of code
      points that are unknown (i.e., not yet designated) for the version
      of Unicode used by the application.

   This document defines the valid, contextual rule required,
   disallowed, and unassigned rules for the IdentifierClass and
   FreeformClass.  As described under Section 4, profiles of these
   string classes are responsible for defining the width mapping,
   additional mappings, case mapping, normalization, directionality, and
   exclusion rules.

3.2.  IdentifierClass

   Most application technologies need strings that can be used to refer
   to, include, or communicate protocol strings like usernames, file
   names, data feed identifiers, and chatroom names.  We group such
   strings into a class called "IdentifierClass" having the following
   features.

3.2.1.  Valid

   o  Code points traditionally used as letters and numbers in writing
      systems, i.e., the LetterDigits ("A") category first defined in
      [RFC5892] and listed here under Section 7.1.

   o  Code points in the range U+0021 through U+007E, i.e., the
      (printable) ASCII7 ("K") rule defined under Section 7.11.  These
      code points are "grandfathered" into PRECIS and thus are valid



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      even if they would otherwise be disallowed according to the
      property-based rules specified in the next section.

      Note: Although the PRECIS IdentifierClass re-uses the LetterDigits
      category from IDNA2008, the range of characters allowed in the
      IdentifierClass is wider than the range of characters allowed in
      IDNA2008.  The main reason is that IDNA2008 applies the Unstable
      category before the LetterDigits category, thus disallowing
      uppercase characters, whereas the IdentifierClass does not apply
      the Unstable category.

3.2.2.  Contextual Rule Required

   o  A number of characters from the Exceptions ("F") category defined
      under Section 7.6 (see Section 7.6 for a full list).

   o  Joining characters, i.e., the JoinControl ("H") category defined
      under Section 7.8.

3.2.3.  Disallowed

   o  Old Hangul Jamo characters, i.e., the OldHangulJamo ("I") category
      defined under Section 7.9.

   o  Control characters, i.e., the Controls ("L") category defined
      under Section 7.12.

   o  Ignorable characters, i.e., the PrecisIgnorableProperties ("M")
      category defined under Section 7.13.

   o  Space characters, i.e., the Spaces ("N") category defined under
      Section 7.14.

   o  Symbol characters, i.e., the Symbols ("O") category defined under
      Section 7.15.

   o  Punctuation characters, i.e., the Punctuation ("P") category
      defined under Section 7.16.

   o  Any character that has a compatibility equivalent, i.e., the
      HasCompat ("Q") category defined under Section 7.17.  These code
      points are disallowed even if they would otherwise be valid
      according to the property-based rules specified in the previous
      section.

   o  Letters and digits other than the "traditional" letters and digits
      allowed in IDNs, i.e., the OtherLetterDigits ("R") category
      defined under Section 7.18.



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3.2.4.  Unassigned

   Any code points that are not yet designated in the Unicode character
   set are considered Unassigned for purposes of the IdentifierClass,
   and such code points are to be treated as Disallowed.

3.2.5.  Examples

   As described in the Introduction to this document, the string classes
   do not handle all issues related to string preparation and comparison
   (such as case mapping); instead, such issues are handled at the level
   of profiles.  Examples for two profiles of the IdentifierClass can be
   found in [I-D.ietf-precis-saslprepbis] (the UsernameIdentifierClass
   profile) and in [I-D.ietf-xmpp-6122bis] (the JIDlocalIdentifierClass
   profile).

3.3.  FreeformClass

   Some application technologies need strings that can be used in a
   free-form way, e.g., as a password in an authentication exchange (see
   [I-D.ietf-precis-saslprepbis]) or a nickname in a chatroom (see
   [I-D.ietf-precis-nickname]).  We group such things into a class
   called "FreeformClass" having the following features.

      Security Warning: As mentioned, the FreeformClass prioritizes
      expressiveness over safety; Section 9.3 describes some of the
      security hazards involved with using or profiling the
      FreeformClass.

      Security Warning: Consult Section 9.6 for relevant security
      considerations when strings conforming to the FreeformClass, or a
      profile thereof, are used as passwords.

3.3.1.  Valid

   o  Traditional letters and numbers, i.e., the LetterDigits ("A")
      category first defined in [RFC5892] and listed here under
      Section 7.1.

   o  Letters and digits other than the "traditional" letters and digits
      allowed in IDNs, i.e., the OtherLetterDigits ("R") category
      defined under Section 7.18.

   o  Code points in the range U+0021 through U+007E, i.e., the
      (printable) ASCII7 ("K") rule defined under Section 7.11.

   o  Any character that has a compatibility equivalent, i.e., the
      HasCompat ("Q") category defined under Section 7.17.



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   o  Space characters, i.e., the Spaces ("N") category defined under
      Section 7.14.

   o  Symbol characters, i.e., the Symbols ("O") category defined under
      Section 7.15.

   o  Punctuation characters, i.e., the Punctuation ("P") category
      defined under Section 7.16.

3.3.2.  Contextual Rule Required

   o  A number of characters from the Exceptions ("F") category defined
      under Section 7.6 (see Section 7.6 for a full list).

   o  Joining characters, i.e., the JoinControl ("H") category defined
      under Section 7.8.

3.3.3.  Disallowed

   o  Old Hangul Jamo characters, i.e., the OldHangulJamo ("I") category
      defined under Section 7.9.

   o  Control characters, i.e., the Controls ("L") category defined
      under Section 7.12.

   o  Ignorable characters, i.e., the PrecisIgnorableProperties ("M")
      category defined under Section 7.13.

3.3.4.  Unassigned

   Any code points that are not yet designated in the Unicode character
   set are considered Unassigned for purposes of the FreeformClass, and
   such code points are to be treated as Disallowed.

3.3.5.  Examples

   As described in the Introduction to this document, the string classes
   do not handle all issues related to string preparation and comparison
   (such as case mapping); instead, such issues are handled at the level
   of profiles.  Examples for two profiles of the FreeformClass can be
   found in [I-D.ietf-precis-nickname] (the NicknameFreeformClass
   profile) and in [I-D.ietf-xmpp-6122bis] (the
   JIDresourceIdentifierClass profile).








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4.  Profiles

4.1.  Principles

   This framework document defines the valid, contextual-rule-required,
   disallowed, and unassigned rules for the IdentifierClass and the
   FreeformClass.  A profile of a PRECIS string class MUST define the
   width mapping, additional mappings (if any), case mapping,
   normalization, directionality, and exclusion rules.  A profile MAY
   also restrict the allowable characters above and beyond the
   definition of the relevant PRECIS string class (but MUST NOT add as
   valid any code points or character categories that are disallowed by
   the relevant PRECIS string class).  These matters are discussed in
   the following subsections.

   Profiles of the PRECIS string classes are registered with the IANA as
   described under Section 8.3.  Profile names use the following
   convention: they are of the form "ProfilenameBaseClass", where the
   "Profilename" string is a differentiator and "BaseClass" is the name
   of the PRECIS string class being profiled; for example, the profile
   of the IdentifierClass used for localparts of Jabber Identifiers
   (JIDs) in the Extensible Messaging and Presence Protocol (XMPP) is
   named "JIDlocalIdentifierClass" [I-D.ietf-xmpp-6122bis].

4.1.1.  Width Mapping

   The width mapping rule of a profile specifies whether width mapping
   is performed on fullwidth and halfwidth characters, and how the
   mapping is done.  Typically such mapping consists of mapping
   fullwidth and halfwidth characters, i.e., code points with a
   Decomposition Type of Wide or Narrow, to their decomposition
   mappings; as an example, FULLWIDTH DIGIT ZERO (U+FF10) would be
   mapped to DIGIT ZERO (U+0030).

   The normalization form specified by a profile (see below) has an
   impact on the need for width mapping.  Because width mapping is
   performed as a part of compatibility decomposition, a profile
   employing either normalization form KD (NFKD) or normalization form
   KC (NFKC) does not need to specify width mapping.  However, if
   Unicode normalization form C (NFC) is used then the profile needs to
   specify whether to apply width mapping; in this case, width mapping
   is in general RECOMMENDED because allowing fullwidth and halfwidth
   characters to remain unmapped to their compatibility variants would
   violate the principle of least user surprise.  For more information
   about the concept of width in East Asian scripts within Unicode, see
   Unicode Standard Annex #11 [UAX11].





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4.1.2.  Additional Mappings

   The additional mappings rule of a profile specifies whether
   additional mappings are to be applied, such as mapping of delimiter
   characters and mapping of special characters (e.g., non-ASCII space
   characters to ASCII space or certain characters to nothing).

4.1.3.  Case Mapping

   The case mapping rule of a profile specifies whether case mapping is
   performed (instead of case preservation) on uppercase and titlecase
   characters, and how the mapping is done (e.g., mapping uppercase and
   titlecase characters to their lowercase equivalents).

   If case mapping is desired (instead of case preservation), it is
   RECOMMENDED to use Unicode Default Case Folding as defined in Chapter
   3 of the Unicode Standard [Unicode7.0].

      Note: Unicode Default Case Folding is not designed to handle
      various localization issues (such as so-called "dotless i" in
      several Turkic languages).  The PRECIS mappings document
      [I-D.ietf-precis-mappings] describes these issues in greater
      detail and defines a "local case mapping" method that handles some
      locale-dependent and context-dependent mappings.

   In order to maximize entropy and minimize the potential for false
   positives, it is NOT RECOMMENDED for application protocols to map
   uppercase and titlecase code points to their lowercase equivalents
   when strings conforming to the FreeformClass, or a profile thereof,
   are used in passwords; instead, it is RECOMMENDED to preserve the
   case of all code points contained in such strings and then perform
   case-sensitive comparison.  See also the related discussion in
   [I-D.ietf-precis-saslprepbis].

4.1.4.  Normalization

   The normalization rule of a profile specifies which Unicode
   normalization form (D, KD, C, or KC) is to be applied (see Unicode
   Standard Annex #15 [UAX15] for background information).

   In accordance with [RFC5198], normalization form C (NFC) is
   RECOMMENDED.

4.1.5.  Directionality

   The directionality rule of a profile specifies how to treat strings
   containing left-to-right (LTR) and right-to-left (RTL) characters
   (see Unicode Standard Annex #9 [UAX9]).  A profile usually specifies



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   a directionality rule that restricts strings to be entirely LTR
   strings or entirely RTL strings and defines the allowable sequences
   of characters in LTR and RTL strings.  Possible rules include, but
   are not limited to, (a) considering any string that contains a right-
   to-left code point to be a right-to-left string, or (b) applying the
   "Bidi Rule" from [RFC5893].

   Mixed-direction strings are not directly supported by the PRECIS
   framework itself, since there is currently no widely accepted and
   implemented solution for the safe display of mixed-direction strings.
   An application protocol that uses the PRECIS framework (or an
   extension to the framework) could define better ways to present
   mixed-direction strings; however, that work is outside the scope of
   this framework and would likely require a great deal of careful
   research into the problems of displaying bidirectional text.

4.1.6.  Exclusions

   The exclusions rule of a profile specifies whether the profile
   excludes additional code points or character categories above and
   beyond those excluded by the string class being profiled.  That is, a
   profile MAY do either of the following:

   1.  Exclude specific code points that are allowed by the relevant
       string class.

   2.  Exclude characters matching certain Unicode properties (e.g.,
       math symbols) that are included in the relevant PRECIS string
       class.

   As a result of such exclusions, code points that are defined as valid
   for the PRECIS string class being profiled will be defined as
   disallowed for the profile.

4.2.  Building Application-Layer Constructs

   Sometimes, an application-layer construct does not map in a
   straightforward manner to one of the base string classes or a profile
   thereof.  Consider, for example, the "simple user name" construct in
   the Simple Authentication and Security Layer (SASL) [RFC4422].
   Depending on the deployment, a simple user name might take the form
   of a user's full name (e.g., the user's personal name followed by a
   space and then the user's family name).  Such a simple user name
   cannot be defined as an instance of the IdentifierClass or a profile
   thereof, since space characters are not allowed in the
   IdentifierClass; however, it could be defined using a space-separated
   sequence of IdentifierClass instances, as in the following pseudo-
   ABNF [RFC5234]:



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      fullname = namepart *(1*SP namepart)
      namepart = 1*idpoint
                 ;
                 ; an "idpoint" is a UTF-8 encoded Unicode code point
                 ; that conforms to the PRECIS IdentifierClass

   Similar techniques could be used to define many application-layer
   constructs, say of the form "user@domain" or "/path/to/file".

4.3.  A Note about Spaces

   With regard to the IdentiferClass, the consensus of the PRECIS
   Working Group was that spaces are problematic for many reasons,
   including:

   o  Many Unicode characters are confusable with ASCII space.

   o  Even if non-ASCII space characters are mapped to ASCII space
      (U+0020), space characters are often not rendered in user
      interfaces, leading to the possibility that a human user might
      consider a string containing spaces to be equivalent to the same
      string without spaces.

   o  In some locales, some devices are known to generate a character
      other than ASCII space (such as ZERO WIDTH JOINER, U+200D) when a
      user performs an action like hit the space bar on a keyboard.

   One consequence of disallowing space characters in the
   IdentifierClass might be to effectively discourage their use within
   identifiers created in newer application protocols; given the
   challenges involved in properly handling space characters (especially
   non-ASCII space characters) in identifiers and other protocol
   strings, the Working Group considered this to be a feature, not a
   bug.

   However, the FreeformClass does allow spaces, which enables
   application protocols to define profiles of the FreeformClass that
   are more flexible than any profiles of the IdentifierClass.  In
   addition, as explained in the previous section, application protocols
   can also define application-layer constructs containing spaces.

5.  Order of Operations

   To ensure proper comparison, the following order of operations is
   REQUIRED:

   1.  Width mapping




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   2.  Optionally, additional mappings such as mapping of delimiters
       (e.g., characters such as '@', ':', '/', '+', and '-') and
       special handling of certain characters or classes of characters
       (e.g., mapping of non-ASCII spaces to ASCII space or mapping of
       control characters to nothing); the PRECIS mappings document
       [I-D.ietf-precis-mappings] describes such mappings in more detail

   3.  Case mapping as described under Section 4.1.3 of this document

   4.  Normalization

   5.  Behavioral rules for determining whether a code point is valid,
       allowed under a contextual rule, disallowed, or unassigned

   As already described, the width mapping, additional mappings, case
   mapping, and normalization operations are specified for each profile,
   whereas the behavioral rules are specified for each string class.
   Some of the logic behind this order is provided under Section 4.1.1
   (see also the PRECIS mappings document [I-D.ietf-precis-mappings]).

6.  Code Point Properties

   In order to implement the string classes described above, this
   document does the following:

   1.  Reviews and classifies the collections of code points in the
       Unicode character set by examining various code point properties.

   2.  Defines an algorithm for determining a derived property value,
       which can vary depending on the string class being used by the
       relevant application protocol.

   This document is not intended to specify precisely how derived
   property values are to be applied in protocol strings.  That
   information is the responsibility of the protocol specification that
   uses or profiles a PRECIS string class from this document.  The value
   of the property is to be interpreted as follows.

   PROTOCOL VALID  Those code points that are allowed to be used in any
      PRECIS string class (currently, IdentifierClass and
      FreeformClass).  Code points with this property value are
      permitted for general use in any string class.  The abbreviated
      term "PVALID" is used to refer to this value in the remainder of
      this document.

   SPECIFIC CLASS PROTOCOL VALID  Those code points that are allowed to
      be used in specific string classes.  Code points with this
      property value are permitted for use in specific string classes.



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      In the remainder of this document, the abbreviated term *_PVAL is
      used, where * = (ID | FREE), i.e., either "FREE_PVAL" or
      "ID_PVAL".  In practice, the derived property ID_PVAL is not used
      in this specification, since every ID_PVAL code point is PVALID.

   CONTEXTUAL RULE REQUIRED  Some characteristics of the character, such
      as its being invisible in certain contexts or problematic in
      others, require that it not be used in labels unless specific
      other characters or properties are present.  As in IDNA2008, there
      are two subdivisions of CONTEXTUAL RULE REQUIRED, the first for
      Join_controls (called "CONTEXTJ") and the second for other
      characters (called "CONTEXTO").  A character with the derived
      property value CONTEXTJ or CONTEXTO MUST NOT be used unless an
      appropriate rule has been established and the context of the
      character is consistent with that rule.  The most notable of the
      CONTEXTUAL RULE REQUIRED characters are the Join Control
      characters U+200D ZERO WIDTH JOINER and U+200C ZERO WIDTH NON-
      JOINER, which have a derived property value of CONTEXTJ.  See
      Appendix A of [RFC5892] for more information.

   DISALLOWED  Those code points that are not permitted in any PRECIS
      string class.

   SPECIFIC CLASS DISALLOWED  Those code points that are not to be
      included in a specific string class.  Code points with this
      property value are not permitted in one of the string classes but
      might be permitted in others.  In the remainder of this document,
      the abbreviated term *_DIS is used, where * = (ID | FREE), i.e.,
      either "FREE_DIS" or "ID_DIS".  In practice, the derived property
      FREE_DIS is not used in this specification, since every FREE_DIS
      code point is DISALLOWED.

   UNASSIGNED  Those code points that are not designated (i.e. are
      unassigned) in the Unicode Standard.

   To summarize, the assigned values of the derived property are:

   o  PVALID

   o  FREE_PVAL

   o  CONTEXTJ

   o  CONTEXTO

   o  DISALLOWED

   o  UNASSIGNED



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   The algorithm to calculate the value of the derived property is as
   follows:

   If .cp. .in. Exceptions Then Exceptions(cp);
   Else If .cp. .in. BackwardCompatible Then BackwardCompatible(cp);
   Else If .cp. .in. Unassigned Then UNASSIGNED;
   Else If .cp. .in. ASCII7 Then PVALID;
   Else If .cp. .in. JoinControl Then CONTEXTJ;
   Else If .cp. .in. OldHangulJamo Then DISALLOWED;
   Else If .cp. .in. PrecisIgnorableProperties Then DISALLOWED;
   Else If .cp. .in. Controls Then DISALLOWED;
   Else If .cp. .in. HasCompat Then ID_DIS or FREE_PVAL;
   Else If .cp. .in. LetterDigits Then PVALID;
   Else If .cp. .in. OtherLetterDigits Then ID_DIS or FREE_PVAL;
   Else If .cp. .in. Spaces Then ID_DIS or FREE_PVAL;
   Else If .cp. .in. Symbols Then ID_DIS or FREE_PVAL;
   Else If .cp. .in. Punctuation Then ID_DIS or FREE_PVAL;
   Else DISALLOWED;

   The value of the derived property calculated can depend on the string
   class; for example, if an identifier used in an application protocol
   is defined as profiling the PRECIS IdentifierClass then a space
   character such as U+0020 would be assigned to ID_DIS, whereas if an
   identifier is defined as profiling the PRECIS FreeformClass then the
   character would be assigned to FREE_PVAL.  For the sake of brevity,
   the designation "FREE_PVAL" is used in the code point tables, instead
   of the longer designation "ID_DIS or FREE_PVAL".  In practice, the
   derived properties ID_PVAL and FREE_DIS are not used in this
   specification, since every ID_PVAL code point is PVALID and every
   FREE_DIS code point is DISALLOWED.

   Use of the name of a rule (such as "Exceptions") implies the set of
   code points that the rule defines, whereas the same name as a
   function call (such as "Exceptions(cp)") implies the value that the
   code point has in the Exceptions table.

   The mechanisms described here allow determination of the value of the
   property for future versions of Unicode (including characters added
   after Unicode 5.2 or 7.0 depending on the category, since some
   categories in this document are reused from IDNA2008 and therefore
   were defined at the time of Unicode 5.2).  Changes in Unicode
   properties that do not affect the outcome of this process therefore
   do not affect this framework.  For example, a character can have its
   Unicode General_Category value (see Chapter 4 of the Unicode Standard
   [Unicode7.0]) change from So to Sm, or from Lo to Ll, without
   affecting the algorithm results.  Moreover, even if such changes were
   to result, the BackwardCompatible list (Section 7.7) can be adjusted
   to ensure the stability of the results.



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7.  Category Definitions Used to Calculate Derived Property

   The derived property obtains its value based on a two-step procedure:

   1.  Characters are placed in one or more character categories either
       (1) based on core properties defined by the Unicode Standard or
       (2) by treating the code point as an exception and addressing the
       code point based on its code point value.  These categories are
       not mutually exclusive.

   2.  Set operations are used with these categories to determine the
       values for a property specific to a given string class.  These
       operations are specified under Section 6.

      Note: Unicode property names and property value names might have
      short abbreviations, such as "gc" for the General_Category
      property and "Ll" for the Lowercase_Letter property value of the
      gc property.

   In the following specification of character categories, the operation
   that returns the value of a particular Unicode character property for
   a code point is designated by using the formal name of that property
   (from the Unicode PropertyAliases.txt [1]) followed by '(cp)' for
   "code point".  For example, the value of the General_Category
   property for a code point is indicated by General_Category(cp).

   The first ten categories (A-J) shown below were previously defined
   for IDNA2008 and are copied directly from [RFC5892] to ease the
   understanding of how PRECIS handles various characters.  Some of
   these categories are reused in PRECIS and some of them are not;
   however, the lettering of categories is retained to prevent overlap
   and to ease implementation of both IDNA2008 and PRECIS in a single
   software application.  The next eight categories (K-R) are specific
   to PRECIS.

7.1.  LetterDigits (A)

   This category is defined in Secton 2.1 of [RFC5892] and is included
   by reference for use in PRECIS.

7.2.  Unstable (B)

   This category is defined in Secton 2.2 of [RFC5892] but not used in
   PRECIS.







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7.3.  IgnorableProperties (C)

   This category is defined in Secton 2.3 of [RFC5892] but not used in
   PRECIS.

   Note: See the "PrecisIgnorableProperties (M)" category below for a
   more inclusive category used in PRECIS identifiers.

7.4.  IgnorableBlocks (D)

   This category is defined in Secton 2.4 of [RFC5892] but not used in
   PRECIS.

7.5.  LDH (E)

   This category is defined in Secton 2.5 of [RFC5892] but not used in
   PRECIS.

   Note: See the "ASCII7 (K)" category below for a more inclusive
   category used in PRECIS identifiers.

7.6.  Exceptions (F)

   This category is defined in Secton 2.6 of [RFC5892] and is included
   by reference for use in PRECIS.

7.7.  BackwardCompatible (G)

   This category is defined in Secton 2.7 of [RFC5892] and is included
   by reference for use in PRECIS.

   Note: Because of how the PRECIS string classes are defined, only
   changes that would result in code points being added to or removed
   from the LetterDigits ("A") category would result in backward-
   incompatible modifications to code point assignments.  Therefore,
   management of this category is handled via the processes specified in
   [RFC5892].  At the time of this writing (and also at the time that
   RFC 5892 was published), this category consisted of the empty set;
   however, that is subject to change as described in RFC 5892.

7.8.  JoinControl (H)

   This category is defined in Secton 2.8 of [RFC5892] and is included
   by reference for use in PRECIS.







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7.9.  OldHangulJamo (I)

   This category is defined in Secton 2.9 of [RFC5892] and is included
   by reference for use in PRECIS.

7.10.  Unassigned (J)

   This category is defined in Secton 2.10 of [RFC5892] and is included
   by reference for use in PRECIS.

7.11.  ASCII7 (K)

   This PRECIS-specific category consists of all printable, non-space
   characters from the 7-bit ASCII range.  By applying this category,
   the algorithm specified under Section 6 exempts these characters from
   other rules that might be applied during PRECIS processing, on the
   assumption that these code points are in such wide use that
   disallowing them would be counter-productive.

   K: cp is in {0021..007E}

7.12.  Controls (L)

   L: Control(cp) = True

7.13.  PrecisIgnorableProperties (M)

   This PRECIS-specific category is used to group code points that are
   discouraged from use in PRECIS string classes.

   M: Default_Ignorable_Code_Point(cp) = True or
      Noncharacter_Code_Point(cp) = True

   The definition for Default_Ignorable_Code_Point can be found in the
   DerivedCoreProperties.txt [2] file, and at the time of Unicode 7.0 is
   as follows:

     Other_Default_Ignorable_Code_Point
   + Cf (Format characters)
   + Variation_Selector
   - White_Space
   - FFF9..FFFB (Annotation Characters)
   - 0600..0604, 06DD, 070F, 110BD (exceptional Cf characters
                                    that should be visible)







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7.14.  Spaces (N)

   This PRECIS-specific category is used to group code points that are
   space characters.

   N: General_Category(cp) is in {Zs}

7.15.  Symbols (O)

   This PRECIS-specific category is used to group code points that are
   symbols.

   O: General_Category(cp) is in {Sm, Sc, Sk, So}

7.16.  Punctuation (P)

   This PRECIS-specific category is used to group code points that are
   punctuation characters.

   P: General_Category(cp) is in {Pc, Pd, Ps, Pe, Pi, Pf, Po}

7.17.  HasCompat (Q)

   This PRECIS-specific category is used to group code points that have
   compatibility equivalents as explained in Chapter 2 and Chapter 3 of
   the Unicode Standard [Unicode7.0].

   Q: toNFKC(cp) != cp

   The toNFKC() operation returns the code point in normalization form
   KC.  For more information, see Section 5 of Unicode Standard Annex
   #15 [UAX15].

7.18.  OtherLetterDigits (R)

   This PRECIS-specific category is used to group code points that are
   letters and digits other than the "traditional" letters and digits
   grouped under the LetterDigits (A) class (see Section 7.1).

   R: General_Category(cp) is in {Lt, Nl, No, Me}

8.  IANA Considerations

8.1.  PRECIS Derived Property Value Registry

   IANA is requested to create a PRECIS-specific registry with the
   Derived Properties for the versions of Unicode that are released
   after (and including) version 7.0.  The derived property value is to



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   be calculated in cooperation with a designated expert [RFC5226]
   according to the rules specified under Section 7 and Section 6.

   The IESG is to be notified if backward-incompatible changes to the
   table of derived properties are discovered or if other problems arise
   during the process of creating the table of derived property values
   or during expert review.  Changes to the rules defined under
   Section 7 and Section 6 require IETF Review.

8.2.  PRECIS Base Classes Registry

   IANA is requested to create a registry of PRECIS string classes.  In
   accordance with [RFC5226], the registration policy is "RFC Required".

   The registration template is as follows:

   Base Class:  [the name of the PRECIS string class]

   Description:  [a brief description of the PRECIS string class and its
      intended use, e.g., "A sequence of letters, numbers, and symbols
      that is used to identify or address a network entity."]

   Specification:  [the RFC number]

   The initial registrations are as follows:

   Base Class: FreeformClass.
   Description: A sequence of letters, numbers, symbols, spaces, and
         other code points that is used for free-form strings.
   Specification: Section 3.3 of this document.
                  [Note to RFC Editor: please change "this document"
                  to the RFC number issued for this specification.]

   Base Class: IdentifierClass.
   Description: A sequence of letters, numbers, and symbols that is
         used to identify or address a network entity.
   Specification: Section 3.2 of this document.
                  [Note to RFC Editor: please change "this document"
                  to the RFC number issued for this specification.]

8.3.  PRECIS Profiles Registry

   IANA is requested to create a registry of profiles that use the
   PRECIS string classes.  In accordance with [RFC5226], the
   registration policy is "Expert Review".  This policy was chosen in
   order to ease the burden of registration while ensuring that
   "customers" of PRECIS receive appropriate guidance regarding the




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   sometimes complex and subtle internationalization issues related to
   profiles of PRECIS string classes.

   The registration template is as follows:

   Name:  [the name of the profile]

   Applicability:  [the specific protocol elements to which this profile
      applies, e.g., "Localparts in XMPP addresses."]

   Base Class:  [which PRECIS string class is being profiled]

   Replaces:  [the Stringprep profile that this PRECIS profile replaces,
      if any]

   Width Mapping:  [the behavioral rule for handling of width, e.g.,
      "Map fullwidth and halfwidth characters to their compatibility
      variants."]

   Additional Mappings:  [any additional mappings are required or
      recommended, e.g., "Map non-ASCII space characters to ASCII
      space."]

   Case Mapping:  [the behavioral rule for handling of case, e.g.,
      "Unicode Default Case Folding"]

   Normalization:  [which Unicode normalization form is applied, e.g.,
      "NFC"]

   Directionality:  [the behavioral rule for handling of right-to-left
      code points, e.g., "The 'Bidi Rule' defined in RFC 5893 applies."]

   Exclusions:  [a brief description of the specific code points or
      characters categories are excluded, e.g., "Eight legacy characters
      in the ASCII range" or "Any character that has a compatibility
      equivalent, i.e., the HasCompat category"]

   Enforcement:  [which entities enforce the rules, and when that
      enforcement occurs during protocol operations]

   Specification:  [a pointer to relevant documentation, such as an RFC
      or Internet-Draft]

   In order to request a review, the registrant shall send a completed
   template to the precis@ietf.org list or its designated successor.

   Factors to focus on while defining profiles and reviewing profile
   registrations include the following:



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   o  Is the problem being addressed by this profile well-defined?

   o  Does the specification define what kinds of applications are
      involved and the protocol elements to which this profile applies?

   o  Would an existing PRECIS string class or profile solve the
      problem?

   o  Is the profile clearly defined?

   o  Is the profile based on an appropriate dividing line between user
      interface (culture, context, intent, locale, device limitations,
      etc.) and the use of conformant strings in protocol elements?

   o  Are the width mapping, case mapping, additional mappings,
      normalization, exclusion, and directionality rules appropriate for
      the intended use?

   o  Does the profile explain which entities enforce the rules, and
      when such enforcement occurs during protocol operations?

   o  Does the profile reduce the degree to which human users could be
      surprised or confused by application behavior (the "principle of
      least user surprise")?

   o  Does the profile introduce any new security concerns such as those
      described under Section 9 of this document (e.g., false positives
      for authentication or authorization)?

9.  Security Considerations

9.1.  General Issues

   If input strings that appear "the same" to users are programmatically
   considered to be distinct in different systems, or if input strings
   that appear distinct to users are programmatically considered to be
   "the same" in different systems, then users can be confused.  Such
   confusion can have security implications, such as the false positives
   and false negatieves discussed in [RFC6943].  One starting goal of
   work on the PRECIS framework was to limit the number of times that
   users are confused (consistent with the "principle of least
   astonishment").  Unfortunately, this goal has been difficult to
   achieve given the large number of application protocols already in
   existence, each with its own conventions regarding allowable
   characters (see for example [I-D.saintandre-username-interop] with
   regard to various username constructs).  Despite these difficulties,
   profiles should not be multiplied beyond necessity.  In particular,
   application protocol designers should think long and hard before



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   defining a new profile instead of using one that has already been
   defined, and if they decide to define a new profile then they should
   clearly explain their reasons for doing so.

   The security of applications that use this framework can depend in
   part on the proper preparation and comparison of internationalized
   strings.  For example, such strings can be used to make
   authentication and authorization decisions, and the security of an
   application could be compromised if an entity providing a given
   string is connected to the wrong account or online resource based on
   different interpretations of the string.

   Specifications of application protocols that use this framework are
   strongly encouraged to describe how internationalized strings are
   used in the protocol, including the security implications of any
   false positives and false negatives that might result from various
   comparison operations.  For some helpful guidelines, refer to
   [RFC6943], [RFC5890], [UTR36], and [UTS39].

9.2.  Use of the IdentifierClass

   Strings that conform to the IdentifierClass and any profile thereof
   are intended to be relatively safe for use in a broad range of
   applications, primarily because they include only letters, digits,
   and "grandfathered" non-space characters from the ASCII range; thus
   they exclude spaces, characters with compatibility equivalents, and
   almost all symbols and punctuation marks.  However, because such
   strings can still include so-called confusable characters (see
   Section 9.5), protocol designers and implementers are encouraged to
   pay close attention to the security considerations described
   elsewhere in this document.

9.3.  Use of the FreeformClass

   Strings that conform to the FreeformClass and many profiles thereof
   can include virtually any Unicode character.  This makes the
   FreeformClass quite expressive, but also problematic from the
   perspective of possible user confusion.  Protocol designers are
   hereby warned that the FreeformClass contains codepoints they might
   not understand, and are encouraged to profile the IdentifierClass
   wherever feasible; however, if an application protocol requires more
   code points than are allowed by the IdentifierClass, protocol
   designers are encouraged to define a profile of the FreeformClass
   that restricts the allowable code points as tightly as possible.
   (The PRECIS Working Group considered the option of allowing
   superclasses as well as profiles of PRECIS string classes, but
   decided against allowing superclasses to reduce the likelihood of
   security and interoperability problems.)



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9.4.  Local Character Set Issues

   When systems use local character sets other than ASCII and Unicode,
   this specification leaves the problem of converting between the local
   character set and Unicode up to the application or local system.  If
   different applications (or different versions of one application)
   implement different rules for conversions among coded character sets,
   they could interpret the same name differently and contact different
   application servers or other network entities.  This problem is not
   solved by security protocols, such as Transport Layer Security (TLS)
   [RFC5246] and the Simple Authentication and Security Layer (SASL)
   [RFC4422], that do not take local character sets into account.

9.5.  Visually Similar Characters

   Some characters are visually similar and thus can cause confusion
   among humans.  Such characters are often called "confusable
   characters" or "confusables".

   The problem of confusable characters is not necessarily caused by the
   use of Unicode code points outside the ASCII range.  For example, in
   some presentations and to some individuals the string "ju1iet"
   (spelled with DIGIT ONE, U+0031, as the third character) might appear
   to be the same as "juliet" (spelled with LATIN SMALL LETTER L,
   U+006C), especially on casual visual inspection.  This phenomenon is
   sometimes called "typejacking".

   However, the problem is made more serious by introducing the full
   range of Unicode code points into protocol strings.  For example, the
   characters U+13DA U+13A2 U+13B5 U+13AC U+13A2 U+13AC U+13D2 from the
   Cherokee block look similar to the ASCII characters "STPETER" as they
   might appear when presented using a "creative" font family.

   In some examples of confusable characters, it is unlikely that the
   average human could tell the difference between the real string and
   the fake string.  (Indeed, there is no programmatic way to
   distinguish with full certainty which is the fake string and which is
   the real string; in some contexts, the string formed of Cherokee
   characters might be the real string and the string formed of ASCII
   characters might be the fake string.)  Because PRECIS-compliant
   strings can contain almost any properly-encoded Unicode code point,
   it can be relatively easy to fake or mimic some strings in systems
   that use the PRECIS framework.  The fact that some strings are easily
   confused introduces security vulnerabilities of the kind that have
   also plagued the World Wide Web, specifically the phenomenon known as
   phishing.





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   Despite the fact that some specific suggestions about identification
   and handling of confusable characters appear in the Unicode Security
   Considerations [UTR36] and the Unicode Security Mechanisms [UTS39],
   it is also true (as noted in [RFC5890]) that "there are no
   comprehensive technical solutions to the problems of confusable
   characters".  Because it is impossible to map visually similar
   characters without a great deal of context (such as knowing the font
   families used), the PRECIS framework does nothing to map similar-
   looking characters together, nor does it prohibit some characters
   because they look like others.

   Nevertheless, specifications for application protocols that use this
   framework are strongly encouraged to describe how confusable
   characters can be abused to compromise the security of systems that
   use the protocol in question, along with any protocol-specific
   suggestions for overcoming those threats.  In particular, software
   implementations and service deployments that use PRECIS-based
   technologies are strongly encouraged to define and implement
   consistent policies regarding the registration, storage, and
   presentation of visually similar characters.  The following
   recommendations are appropriate:

   1.  An application service SHOULD define a policy that specifies the
       scripts or blocks of characters that the service will allow to be
       registered (e.g., in an account name) or stored (e.g., in a file
       name).  Such a policy SHOULD be informed by the languages and
       scripts that are used to write registered account names; in
       particular, to reduce confusion, the service SHOULD forbid
       registration or storage of strings that contain characters from
       more than one script and SHOULD restrict registrations to
       characters drawn from a very small number of scripts (e.g.,
       scripts that are well-understood by the administrators of the
       service, to improve manageability).

   2.  User-oriented application software SHOULD define a policy that
       specifies how internationalized strings will be presented to a
       human user.  Because every human user of such software has a
       preferred language or a small set of preferred languages, the
       software SHOULD gather that information either explicitly from
       the user or implicitly via the operating system of the user's
       device.  Furthermore, because most languages are typically
       represented by a single script or a small set of scripts, and
       because most scripts are typically contained in one or more
       blocks of characters, the software SHOULD warn the user when
       presenting a string that mixes characters from more than one
       script or block, or that uses characters outside the normal range
       of the user's preferred language(s).  (Such a recommendation is
       not intended to discourage communication across different



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       communities of language users; instead, it recognizes the
       existence of such communities and encourages due caution when
       presenting unfamiliar scripts or characters to human users.)

   The challenges inherent in supporting the full range of Unicode code
   points have in the past led some to hope for a way to
   programmatically negotiate more restrictive ranges based on locale,
   script, or other relevant factors, to tag the locale associated with
   a particular string, etc.  As a general-purpose internationalization
   technology, the PRECIS framework does not include such mechanisms.

9.6.  Security of Passwords

   Two goals of passwords are to maximize the amount of entropy and to
   minimize the potential for false positives.  These goals can be
   achieved in part by allowing a wide range of code points and by
   ensuring that passwords are handled in such a way that code points
   are not compared aggressively.  Therefore, it is NOT RECOMMENDED for
   application protocols to profile the FreeformClass for use in
   passwords in a way that removes entire categories (e.g., by
   disallowing symbols or punctuation).  Furthermore, it is NOT
   RECOMMENDED for application protocols to map uppercase and titlecase
   code points to their lowercase equivalents in such strings; instead,
   it is RECOMMENDED to preserve the case of all code points contained
   in such strings and to compare them in a case-sensitive manner.

   That said, software implementers need to be aware that there exist
   tradeoffs between entropy and usability.  For example, allowing a
   user to establish a password containing "uncommon" code points might
   make it difficult for the user to access a service when using an
   unfamiliar or constrained input device.

   Some application protocols use passwords directly, whereas others
   reuse technologies that themselves process passwords (one example of
   such a technology is the Simple Authentication and Security Layer
   [RFC4422]).  Moreover, passwords are often carried by a sequence of
   protocols with backend authentication systems or data storage systems
   such as RADIUS [RFC2865] and LDAP [RFC4510].  Developers of
   application protocols are encouraged to look into reusing these
   profiles instead of defining new ones, so that end-user expectations
   about passwords are consistent no matter which application protocol
   is used.

   In protocols that provide passwords as input to a cryptographic
   algorithm such as a hash function, the client will need to perform
   proper preparation of the password before applying the algorithm,
   since the password is not available to the server in plaintext form.




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   Further discussion of password handling can be found in
   [I-D.ietf-precis-saslprepbis].

10.  Interoperability Considerations

   Although strings that are consumed in PRECIS-based application
   protocols are often encoded using UTF-8 [RFC3629], the exact encoding
   is a matter for the application protocol that uses PRECIS, not for
   the PRECIS framework.

   It is known that some existing systems are unable to support the full
   Unicode character set, or even any characters outside the ASCII
   range.  If two (or more) applications need to interoperate when
   exchanging data (e.g., for the purpose of authenticating a username
   or password), they will naturally need to have in common at least one
   coded character set (as defined by [RFC6365]).  Establishing such a
   baseline is a matter for the application protocol that uses PRECIS,
   not for the PRECIS framework.

   Three Unicode code points underwent changes in their GeneralCategory
   between Unicode 5.2 (current at the time IDNA2008 was originally
   published) and Unicode 6.0, as described in [RFC6452].  Implementers
   might need to be aware that the treatment of these characters differs
   depending on which version of Unicode is available on the system that
   is using IDNA2008 or PRECIS, and that other such differences are
   possible between the version of Unicode current at the time of this
   writing (7.0) and future versions.

11.  References

11.1.  Normative References

   [RFC20]    Cerf, V., "ASCII format for network interchange", RFC 20,
              October 1969.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, March 2008.

   [Unicode7.0]
              The Unicode Consortium, "The Unicode Standard, Version
              6.0.0", 2014,
              <http://www.unicode.org/versions/Unicode7.0.0/>.






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11.2.  Informative References

   [I-D.ietf-precis-mappings]
              Yoneya, Y. and T. NEMOTO, "Mapping characters for PRECIS
              classes", draft-ietf-precis-mappings-08 (work in
              progress), June 2014.

   [I-D.ietf-precis-nickname]
              Saint-Andre, P., "Preparation and Comparison of
              Nicknames", draft-ietf-precis-nickname-09 (work in
              progress), January 2014.

   [I-D.ietf-precis-saslprepbis]
              Saint-Andre, P. and A. Melnikov, "Username and Password
              Preparation Algorithms", draft-ietf-precis-saslprepbis-07
              (work in progress), March 2014.

   [I-D.ietf-xmpp-6122bis]
              Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Address Format", draft-ietf-xmpp-
              6122bis-12 (work in progress), March 2014.

   [I-D.saintandre-username-interop]
              Saint-Andre, P., "An Interoperable Subset of Characters
              for Internationalized Usernames", draft-saintandre-
              username-interop-03 (work in progress), March 2014.

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)", RFC
              2865, June 2000.

   [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              December 2002.

   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.

   [RFC3491]  Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
              Profile for Internationalized Domain Names (IDN)", RFC
              3491, March 2003.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC4422]  Melnikov, A. and K. Zeilenga, "Simple Authentication and
              Security Layer (SASL)", RFC 4422, June 2006.



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   [RFC4510]  Zeilenga, K., "Lightweight Directory Access Protocol
              (LDAP): Technical Specification Road Map", RFC 4510, June
              2006.

   [RFC4690]  Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
              Recommendations for Internationalized Domain Names
              (IDNs)", RFC 4690, September 2006.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, August 2010.

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891, August 2010.

   [RFC5892]  Faltstrom, P., "The Unicode Code Points and
              Internationalized Domain Names for Applications (IDNA)",
              RFC 5892, August 2010.

   [RFC5893]  Alvestrand, H. and C. Karp, "Right-to-Left Scripts for
              Internationalized Domain Names for Applications (IDNA)",
              RFC 5893, August 2010.

   [RFC5894]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Background, Explanation, and
              Rationale", RFC 5894, August 2010.

   [RFC5895]  Resnick, P. and P. Hoffman, "Mapping Characters for
              Internationalized Domain Names in Applications (IDNA)
              2008", RFC 5895, September 2010.

   [RFC6365]  Hoffman, P. and J. Klensin, "Terminology Used in
              Internationalization in the IETF", BCP 166, RFC 6365,
              September 2011.

   [RFC6452]  Faltstrom, P. and P. Hoffman, "The Unicode Code Points and
              Internationalized Domain Names for Applications (IDNA) -
              Unicode 6.0", RFC 6452, November 2011.



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   [RFC6885]  Blanchet, M. and A. Sullivan, "Stringprep Revision and
              Problem Statement for the Preparation and Comparison of
              Internationalized Strings (PRECIS)", RFC 6885, March 2013.

   [RFC6943]  Thaler, D., "Issues in Identifier Comparison for Security
              Purposes", RFC 6943, May 2013.

   [UAX9]     The Unicode Consortium, "Unicode Standard Annex #9:
              Unicode Bidirectional Algorithm", September 2012,
              <http://unicode.org/reports/tr9/>.

   [UAX11]    The Unicode Consortium, "Unicode Standard Annex #11: East
              Asian Width", September 2012,
              <http://unicode.org/reports/tr11/>.

   [UAX15]    The Unicode Consortium, "Unicode Standard Annex #15:
              Unicode Normalization Forms", August 2012,
              <http://unicode.org/reports/tr15/>.

   [UnicodeCurrent]
              The Unicode Consortium, "The Unicode Standard",
              2014-present, <http://www.unicode.org/versions/latest/>.

   [UTR36]    The Unicode Consortium, "Unicode Technical Report #36:
              Unicode Security Considerations", July 2012,
              <http://unicode.org/reports/tr36/>.

   [UTS39]    The Unicode Consortium, "Unicode Technical Standard #39:
              Unicode Security Mechanisms", July 2012,
              <http://unicode.org/reports/tr39/>.

11.3.  URIs

   [1] http://unicode.org/Public/UNIDATA/PropertyAliases.txt

   [2] http://unicode.org/Public/UNIDATA/DerivedCoreProperties.txt

Appendix A.  Acknowledgements

   The authors would like to acknowledge the comments and contributions
   of the following individuals during working group discussion: David
   Black, Edward Burns, Dan Chiba, Mark Davis, Alan DeKok, Martin
   Duerst, Patrik Faltstrom, Ted Hardie, Joe Hildebrand, Bjoern
   Hoehrmann, Paul Hoffman, Jeffrey Hutzelman, Simon Josefsson, John
   Klensin, Alexey Melnikov, Takahiro Nemoto, Yoav Nir, Mike Parker,
   Pete Resnick, Andrew Sullivan, Dave Thaler, Yoshiro Yoneya, and
   Florian Zeitz.




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   Charlie Kaufman, Tom Taylor, and Tim Wicinski reviewed the document
   on behalf of the Security Directorate, the General Area Review Team,
   and the Operations and Management Directorate, respectively.

   During IESG review, Alissa Cooper, Stephen Farrell, and Barry Leiba
   provided comments that led to further improvements.

   Some algorithms and textual descriptions have been borrowed from
   [RFC5892].  Some text regarding security has been borrowed from
   [RFC5890], [I-D.ietf-precis-saslprepbis], and
   [I-D.ietf-xmpp-6122bis].

   Peter Saint-Andre wishes to acknowledge Cisco Systems, Inc., for
   employing him during his work on earlier versions of this document.

Authors' Addresses

   Peter Saint-Andre
   &yet
   P.O. Box 787
   Parker, CO  80134
   USA

   Email: peter@andyet.net


   Marc Blanchet
   Viagenie
   246 Aberdeen
   Quebec, QC  G1R 2E1
   Canada

   Email: Marc.Blanchet@viagenie.ca
   URI:   http://www.viagenie.ca/

















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